1. Field of the Invention
The present invention generally relates to the field of voltage controlled amplifiers (VCAs), and more specifically to a negative resistance circuit for reducing distortion in a VCA.
2. Description of the Related Art
Non-linearity distortion is a substantial cause of error in a wide-band amplifier. The mechanisms which contribute to non-linearity distortion are inherent in the physical properties of the semiconductor pn junctions in bipolar transistors. The signal-amplitude error in an uncompensated wide-band amplifier may be as high as one order of magnitude. Many prior art attempts have been made to reduce the non-linearity of this type of differential amplifier.
A conventional differential amplifier includes a differential stage having a pair of emitter-coupled transistors. A pair of emitter resistors are connected between the respective transistors' emitters and a current source, and a pair of gain resistors are connected between the respective transistors' collectors and a supply voltage. The differential amplifier responds to a differential input voltage applied at the bases of the emitter-coupled transistors by apportioning the source current between the two sides of the differential stage to produce a differential output voltage between their collectors.
Ideally, the differential output voltage is a linear function of the differential input voltage. However, the base-emitter voltages of the emitter-coupled transistors are logarithmic functions of the currents that flow through the respective sides of the differential amplifier, and hence are only equal when the differential input voltage is zero. Therefore, the difference between the base-emitter voltages is manifested as a non-linearity distortion in the differential output voltage.
U.S. Pat. No. 4,146,844 to Quinn, discloses a feed-forward technique to provide a first order correction of amplifier distortion. In FIG. 4 of Quinn, an additional cascode stage consisting of a pair of base connected transistors is connected to the primary differential stage so that the differential voltage at an output of the differential stage is equal to the difference in the base-emitter voltages of the differential stage transistors. A correction amplifier is coupled to the primary differential stage to sense base-emitter distortion through the differential voltage and develop an error signal which, in turn, is coupled to the output of the primary differential stage to cancel any distortion which may be present.
Quinn's primary differential stage includes a pair of emitter-coupled transistors which receive differential signals at their respective bases. The correction amplifier is a second differential stage of emitter-coupled transistors. The bases of the transistors in the second differential stage are coupled to the collectors of the transistors in the primary differential stage. The differential configuration of the correction amplifier reduces the distortion present in the primary differential stage. However, this configuration also introduces a phase distortion analogous to the distortion caused by a feed-back loop.
Sansen and Meyer, "Distortion in bipolar Transistor Variable-Gain Amplifiers," IEEE Journal Solid-State Circuits, vol. SC-8, pp. 275-282, August 1973 disclose a variable gain amplifier 10, commonly known as a VCA, as shown in FIG. 1. VCAs are commonly used in audio systems to provide variable signal compression. The VCA 10 includes a differential stage 12 that responds to a differential voltage V.sub.in to produce currents I.sub.1 and I.sub.2. The difference between I.sub.2 and I.sub.1 forming a differential signal current i.sub.in. A gain control stage 14 apportions I.sub.1 and I.sub.2 between respective pairs of gain transistors in response to a variable control voltage V.sub.c to set the gain of the VCA and produce a differential output voltage V.sub.out.
The differential stage 12 includes a pair of emitter-coupled transistors Q1 and Q2. A pair of emitter resistors, which are both shown as R.sub.E, are connected between the respective emitters 16 and 18 of Q1 and Q2 and a current source 20, which is connected to a low reference voltage V.sub.EE and supplies bias current I.sub.E. The emitter resistors establish the gain of the differential stage 12 and determine the range of differential input voltages over which the emitter-coupled pair behaves approximately as a linear amplifier.
Differential stage 12 responds to the application of differential input signals -V.sub.in /2 and V.sub.in /2 at the bases of transistors Q1 and Q2, respectively, by modulating currents I.sub.1 =I.sub.E /2-i.sub.in /2 and I.sub.2 =I.sub.E /2+i.sub.in /2 that flow through Q1 and Q2, respectively, where I.sub.E /2 is the bias component and .+-.i.sub.in /2 are the differential components of the currents. Alternately, one of the bases can be connected to a reference voltage, typically ground, and a single-ended input V.sub.in can be applied to the other base. When V.sub.in equals zero, the current I.sub.E supplied by current source 20 is split equally between both sides of differential stage 12 so that I.sub.1 =I.sub.2 and i.sub.in is zero.
In general, the differential stage 12 responds to the differential input voltage Vin by increasing the current that flows through its relatively positive side and by reducing the current that flows through its relatively negative side by an equal amount so that I.sub.1 and I.sub.2 are imbalanced. Employing a first order analysis and assuming that the base currents of Q1 and Q2 can be ignored, the relationship between I.sub.2 and I.sub.1 can be described as follows: EQU I.sub.2 -I.sub.1 =i.sub.in =V.sub.in /Re+(V.sub.be (Q.sub.1)-V.sub.be (Q2))/R.sub.E ( 1)
The gain control stage 14 includes two pair of emitter-coupled transistors Q3, Q4 and Q5, Q6, respectively. The emitters 22 and 24 of Q3 and Q4, respectively, are connected to the collector 26 of Q1, and together supply current I.sub.1. Similarly, the emitters 28 and 30 of Q5 and Q6, respectively, are connected to the collector 32 of Q2, and together supply current I.sub.2. As shown, the bases 34 and 36 of transistors Q3 and Q6 are connected to a reference voltage, typically ground.
A variable gain control voltage V.sub.c is applied to the bases 38 and 40 of transistors Q4 and Q5, which are connected together, causing the currents I.sub.1 and I.sub.2 to be split into currents I.sub.3 and I.sub.4, and I.sub.5 and I.sub.6, respectively, where I.sub.4 =GI.sub.1, I.sub.3 =(1-G)I.sub.1, I.sub.5 =GI.sub.2 and I.sub.6 =(1-G)I.sub.2. The splitting fraction G varies between 0 and 1 in response to the gain control voltage V.sub.c according to the following relation: ##EQU1## where V.sub.t is the transistor's thermal voltage ##EQU2## Alternately, the gain control voltage V.sub.c could be applied differentially between the bases of Q4 and Q5, and Q3 and Q6.
The collectors 42 and 44 of transistors Q3 and Q6, respectively, are connected directly to a high voltage supply V.sub.cc, and the collectors 46 and 48 of Q4 and Q5, respectively, are connected through a pair of load resistors R.sub.L to V.sub.cc. A differential output voltage V.sub.out is taken between output terminals A and B at the collectors 46 and 48 of Q4 and Q5, respectively, and can be characterized as follows: EQU V.sub.out =V.sub.B -V.sub.A =-GR.sub.g (I.sub.2 -I.sub.1) (3)
Combining equations 1 and 3 gives: ##EQU3## The term ##EQU4## represents the desired linear response to the differential input voltage. The term ##EQU5## is manifested as an odd-order non-linearity distortion in the differential output voltage, and is the result of the inherent distortion characteristics of transistors Q1 and Q2.
To reduce the odd-order non-linearity distortion, Sansen and Meyer reconfigure the gain control stage, as shown in their FIG. 4, so that the collectors of Q3 and Q5 are connected together to one of the load resistors R.sub.L and the collectors of Q4 and Q6 are connected together to the other load resistor. The output voltage V.sub.out is then taken between the collectors of Q5 and Q6, which reduces the odd-order distortion.